How to select the fcurve of a material
The action editor that you are using only shows a subset of keyframed values, to see all available keyed data you need to use the full dopesheet or the graph editor.



If the values you are looking for are still not visible then check the options in the header that filter the displayed items.



EDIT:

An action is basically a container for animation data that can be turned into an NLA strip. One active action is associated with an object for editing and is cleared when added to the NLA, it can manually be activated again in the action editor if further editing is needed. The dopesheet and graph editor only show animation data that is in the current action for an object.

A materials animation data is stored in it's own action, but this action is not shown in the action editor and only material animation data that is associated with a material in a selected object is shown in the dopesheet and graph editor. Once a materials action is turned into an NLA strip blender does't provided a simple way to re-activate it for further editing. You need to use python to re-activeate a materials action so that it can be edited.

In blender's python console enter the following (adjusting the names) -

bpy.data.materials['Material'].animation_data.action = bpy.data.actions['Material.action']


Then the material animation data can be edited in the dopesheet or graph editor.